Conventionally, as an eccentrically oscillating gear unit for driving robot members and the like, known is an eccentrically oscillating gear unit like the type described in Japanese Patent Application Publication No. H8-226498. This kind of gear unit comprises an outer cylinder, a plurality of internal gear pins, a carrier, a crank shaft, and a plurality of oscillating gears. The plurality of internal gear pins are mounted along the axial direction of the outer cylinder on the inner peripheral face of the outer cylinder. The carrier is housed inside the outer cylinder. The crank shaft is rotatably supported by the carrier. The plurality of oscillating gears are supported by the carrier so as to oscillate in conjunction with the rotation of the crank shaft, and engage with the internal gear pin.
With this kind of gear unit, the respective eccentric parts of the crank shaft that eccentrically oscillates the oscillating gear are engaged with through-holes formed on the oscillating gear via a roller bearing such as a needle bearing.
The needle bearing is provided to the respective eccentric parts of the crank shaft. The needle bearing is supported so that the oscillating gear becomes relatively rotatable in relation to the eccentric parts of the crank shaft. The needle bearing comprises a plurality of needle rollers, and a retainer which rollably supports the needle roller. The needle roller comprises a narrow cylindrical roller.
The end faces of the retainer of the adjacent needle bearings are in direct contact with each other. Consequently, movement of the crank shaft in the axial direction in the needle bearing is regulated.
Nevertheless, with this kind of structure in which the end faces of the retainers are in direct contact, when the eccentricity in which the eccentric parts in the crank shaft become decentered from the shaft part increases, the contact area of the retainers decreases and, consequently, the contact portion across the circumferential direction of the retainers becomes small. As a result, there is a possibility that this may cause the early breakage of the retainer and the needle bearing containing the retainer.
Meanwhile, with the gear unit described in Japanese Patent Application Publication No. H5-180277, the retainers are not in direct contact, a disk-shaped flange is provided between the retainers, and movement of the needle bearing in the axial direction is regulated by the flange. The flange is provided integrally with the crank shaft between the eccentric parts in the crank shaft.
With the gear unit described in Japanese Patent Application Publication No. H5-180277, the flange is provided at a position outside the axial direction relative to the oscillating gear between the eccentric parts of the crank shaft. Thus, with this crank shaft, the gap between the eccentric parts needs to be broadened in the amount of the thickness of the flange and, consequently, the crank shaft needs to be extended in the axial direction. Thus, it may become difficult to downsize the gear unit.